1. Field of the Invention
This invention relates to an electroluminescent lamp (hereinafter referred to as an xe2x80x9cEL lampxe2x80x9d).
2. Description of the Related Art
EL lamps in general allow a luminescent body inside a luminescent layer to emit rays of light by an alternating electric field by laminating the luminescent layer and an insulating layer between a transparent electrode and a rear electrode. A multi-layered EL lamp is known that includes a plurality of laminates each comprising the transparent electrode, the luminescent layer and the insulating layer, and allows these laminates to emit the rays of light either independently or simultaneously in a plane of the multi-layered EL lamp. A multi-color multi-layered EL lamp having the two-layered structure, that is disclosed in Japanese Patent No. 2,696,056, is one of the EL lamps of this kind.
Generally, when the multi-layered EL lamp comprises two laminates, luminescence of one, or both, of a first laminate (front surface side) and a second laminate (rear surface side) constituting the EL lamp is viewed from either one of the surface sides. Luminescence of the rear surface side can be viewed as luminescence passing through the laminate disposed on the front surface side and vice-versa. Therefore, if luminance from each laminate is equal, the luminance when viewed from a particular side naturally appears different between the case where the front surface side is allowed to emit light and the case where the rear surface side is allowed to emit light.
When the thickness of the laminate on the front surface side, for example, is decreased to reduce the difference of luminance between the front surface side and the rear surface side of the laminates in the multi-layered EL lamp when viewed from the front, or to prevent as much as possible the rays of light of the rear surface side from being intercepted by the laminate on the front surface side, the quantity of transmitting light from the rear surface side when viewed from the front increases. However, because the constituent film of the laminate on the front surface side is thin, an impressed voltage of the luminescent layer on the front surface side increases, and luminescence of the front surface side itself increases. Consequently, the difference of luminescence of both laminates as viewed from the front surface side cannot be decreased. Further, when the thickness of the laminate on the front surface side is decreased, deterioration on the front surface side is promoted, causing a difference of service life between the laminate on the front surface side and the laminate on the rear surface side.
To solve the problems described above, the present invention makes luminance of laminates of a multi-layered EL lamp different between the front surface side and the rear surface side. Namely, the present invention sets a dielectric constant for emitting light on the front surface side to a value smaller than a dielectric constant for emitting light on the rear surface side so that the difference of luminance between the front surface side and the rear surface side as viewed from the front surface side can be decreased. The present invention sets such a difference of the dielectric constants by adjusting mixing ratios of a high dielectric material to be mixed in the laminates constituting the EL lamp on the front and rear surface sides, or by changing the thickness of respective luminescent layers.
An EL lamp according to the present invention comprises a first laminate formed by laminating serially a first transparent electrode, a first luminescent layer and a first insulating layer, a second laminate formed by laminating serially a second transparent electrode, a second luminescent layer and a second insulating layer on the first laminate, and a rear electrode formed on the second laminate, wherein a dielectric constant between the first transparent electrode and the second transparent electrode is smaller than a dielectric constant between the second transparent electrode and the rear electrode.
To set the dielectric constant between the first transparent electrode and the second transparent electrode to a value smaller than the dielectric constant between the second transparent electrode and the rear electrode, the amount of the high dielectric material to be mixed in the first insulating layer is preferably not greater than 90% of the amount of the high dielectric material to be mixed in the second insulating layer.
To set the dielectric constant between the first transparent electrode and the second transparent electrode to a value smaller than the dielectric constant between the second transparent electrode and the rear electrode, the thickness of the first luminescent layer is preferably 130 to 250% of the thickness of the second luminescent layer.
Furthermore, the thickness of the first insulating layer is preferably not greater than 90% of the thickness of the second insulating layer to set the dielectric constant between the first transparent electrode and the second transparent electrode to a value smaller than the dielectric constant between the second transparent electrode and the rear electrode, and to improve transmission luminance of the second laminate that can be viewed through the first laminate.